Solvent refining of hydrocarbon oil



July 16, 1940- w. E. SKELTQN SOLVENT REFINING OF HYDROCARBON OIL Filed May 25, 1938 O T *3 K E 5T 1N T E W M m n WI T W A N R f d M O Y s B EX TRACT PHASE EXTRACTION TOWER on. CHARGE Patented July 16, 1940 UNlTED STATES PATENT orrics William E. Skelton, Beacon, N. Y., assignor to The Texas Company, New York, N. Y., a corporation of Delaware Application May 25, 1938, Serial No. 209,861

2 Claims.

This invention relates to the solvent refining of hydrocarbon oil and more particularly to extraction of fractions of mineral oil with a selective solvent.

The invention contemplates a method of countercurrently extracting oil in an extraction zone wherein the portion of the extraction zone from which the raflinate is removed is maintained under temperatures substantially above that of complete miscibility between the solvent and the charge oil undergoing treatment. The temperature so maintained may be substantially above the normal boiling point of the solvent and therefore it is contemplated maintaining the extraction system under pressure sufficient to maintain both the oil and solvent in a liquid condition within the extraction zone.

It is contemplated reducing the vapor pressure effect of the solvent by commingling a portion of the rafiinate with the entering solvent, thus recycling the raffinate through the solvent feed portion of the extraction zone. The invention thus contemplates refluxing to the solvent feed portion of the extraction zone a portion of the rafiinate phase.

The method of this invention has application to the extraction of mineral oils such as lubricating oil or other petroleum fractions with solvents such as furfural, nitrobenzene, phenol, chlorphenol, dichlorethyl ether, etc. It is also applicable to refining with solvents lighter than the oil undergoing treatment such as acetone, alcohol, etc.

In the refining of lubricating oil with furfural, it is customary to employ as the extracting means a packed tower through which the oil and solvent flow countercurrently. The solvent is introduced usually at a temperature of around 275 F. to the upper portion of the tower, while the raw charge oil is introduced to the lower portion of the tower at a temperature of around 140 F., for example. Separation of the mixture of oil and solvent into extract and raflinate phases occurs within the tower. The extract phase, rich in so-called naphthenic or low viscosity index constituents, is removed from the bottom of the tower, while the rafiinate phase, rich in so-called paraffinic or high viscosity index constituents, is removed from the top of the tower.

Under the temperature conditions thus maintained, a temperature gradient of around to F. exists throughout the extraction zone. The: top of the tower is usually maintained at a temperature of about 30 F. below the temperature of complete miscibility between oil and solvent.

In accordance with the method of the present invention a portion of the rafiinate phase withdrawn from the top of the tower is continuously commingled with the entering solvent and thus recycled through the solvent feed portion of the extraction tower. Advantageously, the mixture of solvent and recycled rainnate phase is heated to a temperature of around 350 F. or substantially above the temperature of complete miscibility between the solvent and the raffinate oil before introduction to the tower. The raw oil charge is still introduced at a substantially lower temperature, for example, around F. Under these conditions the temperature at the top of the tower ,is maintained within a range of 300 to 350 F., while the temperature at the bottom of the tower is maintained around 1'70 to 250 F.

One object in recycling the raiiinate phase in this way is to reduce the vapor pressure effect of the solvent so that the solvent may be used at temperatures above its normal boiling point without the necessity of maintaining the pressure substantially above atmospheric pressure. For example, when operating with furfural as the selective solvent, in accordance with the method of this invention, the pressure maintained upon the extraction tower would be around 10 to 50 pounds gauge.

Another object is the obtaining of increased refining efficiency by heating the recycled rafiinate with fresh solvent above the temperature existing at the top of the tower. More extract is removed from this raffinate than corresponds to normal conditions at the top tower temperature without recycling, and, consequently, a higher quality of rafiinate is produced. This is believed analogous to the use of reflux in a fractionating tower where the overhead or distillate product is recycled to the top of the tower to obtain a greater degree of fractionation.

As previously stated, the recycled raffinate-phase and solvent are heated to a temperature above that of complete miscibility prior to introduction to the tower. Upon introduction to the tower the mixture is cooled by contact with the cooler liquid within the tower since the raw oil is being charged at a relatively lower temperature to the lower portion of the tower. within the top of the tower is reduced to a temperature below that of complete miscibility. However, by reason of the fact that this recycled rafiinate passes through the equilibrium point between complete miscibility and phase separation, a more effective contact between solvent and raifinate is thereby obtained. Consequently,

Therefore, the mixture there results agreater degree of separation of less paraffinic constituents from the more parafiinic constituents.

The miscibility temperature of the raffinate oil and the solvent is considerably higher than the miscibility temperature of the raw oil charge and the solvent. In other words, it is generally true that as an oil is refined to remove the less paraffinic portion, the miscibility temperature of the raflinate and solvent rises as the refining operation is increased. For example, a raw oil charge to the tower may have a miscibility temperature with furfural as low as 250 F., while the raifinate' discharged from the top of the tower may have a miscibility temperature with the furfural as high as 340 to 350 F.

In order to further illustrate the invention, reference will now be made to the accompanying drawing:

In the drawing the numeral It designates a countercurrent extraction tower provided with suitable packing material such as Raschig rings.

A lubricating oil stock is introduced continuously, as indicated, to the lower portion of the tower through a pipe I I. Furfural is continuously introduced tothe upper portion of the tower through a pipe l2.

Extract and raffinate phases are formed and collect in the bottom and top of the tower, respectively.

The extract phase is withdrawn through a pipe I3, as indicated, while the rafiinate phase is withdrawn through a pipe M, as indicated. By

proper adjustment of valves i5 and IS a suitable, proportion of the withdrawn raffinate phase is continuously introduced to the pipe l2, wherein it is commingled with the fresh entering solvent. The mixture passes through a heater H, where it is heated, as, for example, to a temperature around, or somewhat above, the temperature of complete miscibility.

Pressure may be maintained within the tower by hydrostatic means, in which case a pipe I 8 extends vertically above the top of the tower. The rafinate phase accumulating in the top of the tower rises upwardly through the pipe I8 and supports a float l9, which, in turn, actuates a float-controlled valve 20 in the pipe I4 pre viously mentioned. The length of the pipe I8 will depend upon the amount of pressure necessary to maintain within the extraction tower.

It is contemplated, of course, that other means of controlling the pressure may be employed.

By way of a specific example, lubricating oil distillate, such as derived from Mid-Continent crude, is continuously introduced through the pipe ll, while furfural is similarly introduced through the pipe I 2 in the proportion of'1 to 2 parts of furfural to one part of oil. The raw oil enters at a temperature of around F., while the furfural mixed with the recycled raifinate phase is introduced at a temperature of 350 F. The amountof raffinate recycled will be in the proportion of to 3 parts to one part of raffinate phase discharged from the system.

Underthese conditions the temperature at the bottom of the tower will be around 172 F. or in the range of about to F. The temperature at the top of the tower, and which will correspond to the temperature of the raflinate phase being withdrawn therefrom, will be in the range 300 to 350 F.

The pressure at the top of the tower will range from about 10 to 12 pounds gauge.

The following data illustrate the improvement in yield of rafiinate oil obtained by maintaining a higher temperature at the top of the extraction tower when extracting a lubricating oil distillate derived from Mid-Continent crude and having a Saybolt Universal viscosity at 210 F. of about '70 seconds:

In both instances the solvent dosage was substantially the same but, as indicated, by operating the tower at 319 F. instead of 298 F., and under superatmospheric pressure a very substantial increase of railinate of slightly better viscosity index was obtained.

It is contemplated that instead of recycling a portion of the rafiinate phase, oil from some other source and of substantially the same grade as that of the rafiinate oil, may be commingled with the solvent entering the extraction tower. Where the rafiinate phase oil is recycled it is advantageous in some instances to remove all, or a portion of, the solvent therefrom prior to commingling it with the fresh solvent enterin the extraction tower. The reduction in amount of solvent recycled is beneficial from the standpoint of minimizing the previously discussed vapor pressure efi'ect of the solvent within the tower.

of the invention, as hereinbefor set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. In the treatment of hydrocarbon oil with a selective solvent by continuous countercurrent contact within a vertical tower to form extract and raifinate phases, respectively, said phases accumulating in opposite ends of the tower for continuous discharge therefrom, the steps comprising introducing the feed oil to the section of the tower in which extract phase accumulates, commingling the solvent prior to introduction to the tower with discharged rafiinate phase in the proportion of about one-half to three parts of raflinate phase to one part of solvent, heating the commingled mixture to a temperature of around the boiling point of the solvent and above, and substantially above the temperature of complete miscibility between the solvent and raffinate oil, introducing the so-heated mixture to the tower at a point intermediate the points of oil introduction and rafiinate phase discharge, maintaining the temperature adjacent the rafiinate phase discharge substantially below the temperature of the entering mixture of solvent and raffinate, while maintaining the temperature in the feed oil portion of the tower at a still lower level, and maintaining the extraction tower under pressure sufiicient to keep the solvent and oil in the liquid phase within the tower.

2. In the continuous counterourrent extraction of mineral lubricating oil with a solvent comprising furfural in a vertical tower to form extract and railmate phases, respectively, and which are continuously discharged therefrom, the steps comprising introducing the oil to the lower por- 35 Obviously manymodifications and variations tion of the vertical tower and above the point of extract discharge therefrom, commingling the solvent prior to introduction to the tower with discharged rafilnate phase in the proportion of about one-half to three parts of raffinate phase to one part of solvent, heating the commingled mixture to a temperature of around the boiling point of the solvent and above, and substantially above the temperature of complete miscibility between the solvent and rafiinate oil, introducing the so-heated mixture to the upper portion of the tower and below the point of rafiinate phase discharge, maintaining the temperature at the bottom of the tower in the range 150 to 180 F.,

maintaining the temperature at the top of the tower adjacent the rafiinate discharge at around 300 F. and below the temperature of the enterv ing mixture of solvent and raifinate, and maintaining the tower under pressure sufficient to keep the solvent and oil in the liquid phase within the tower.

' WILLIAM E. SKELTON. 

